The present invention relates to an apparatus for manufacturing alumina. More particularly this invention concerns an apparatus for extracting alumina from bauxite.
There are two main methods of extracting alumina from bauxite, the so-called European and American methods. The European bauxite mainly contains a monohydrate that is used along with concentrated lye to produce a fine-grained aluminum hydrate. In the United States the bauxite mainly contains a trihydrate so that coarse grain aluminum hydrate can be produced using diluted lye.
In both methods the bauxite is ground, mixed with a caustic-sode solution of a specific concentration and agitated in an autoclave. According to the European method a temperature of approximately 240.degree. C. and a pressure of approximately 45 bars is used. With the American method the temperature is approximately 150.degree. C. and the pressure is only 5 bars. Furthermore the concentration in the digestion lye according to the European method is approximately 325 g/l to 600 g/l (expressed as Na.sub.2 CO.sub.3) and in the American method the concentration is normally 160 g/l to 200 g/l. The ratio of the dissolved aluminum oxide in grams per liter to the dissolved caustic sode in grams per liter, normally referred to as the A/C ratio, is normally approximately 0.3 for both methods.
Furthermore in accordance with both methods the mixture of ground bauxite, caustic soda and water is concentrated at a temperature of between 100.degree. C. and 105.degree. C. at atmospheric pressures and the impurities are removed as a so-called red mud. After such a concentration there is left a solution of caustic soda and dissolved alumina. In the European method the red mud is subsequently washed and the washed water which contains some aluminum hydrates is again mixed with the overflow.
In accordance with both of these known methods a supersaturated solution of alumina in sodium hydroxide is obtained. Sodium hydroxide concentration in the European method is somewhat higher than that in the American method, but the A/C ratio according to both methods is approximately 0.6.
This supersaturated solution of alumina and caustic soda, referred to as a mother liquor is now seeded with crystals, normally of aluminum hydrate. In accordance with the European method fine crystals are employed in a quantity of about 200 g/l to 400 g/l. The American method uses fine and medium-coarse seed crystals in a quantity of about 30 g/l to 150 g/l.
Therefter the liquid is agitated and conveyed through a succession of series-connected stirrer-tanks. In accordance with the American method, the temperature is decreased from approximately 80.degree. C. at the furthest upstream stirrer-tank to about 60.degree. C. at the last stirrer-tank, whereas in the European method the temperature drops from 65.degree. C. to approximately 50.degree. C. During agitation part of the dissolved alumina goes into suspension as aluminum hydroxide. The American method produces coarse, medium-coarse, and fine crystals and the European method produces mainly fine crystals.
In accordance with the European method the crystals so produced are separated by means of a single-stage filtration and a portion is returned to the process as seed crystals with the remainder of the crystals being separated out and calcined to alumina which is later on used for the production of aluminum. The American method separates the crystals in a three-stage decantation procedure with the coarse crystals being retained as the product for later production of alumina and the medium coarse and fine crystals being returned to the cycle as seed crystals.
After the separation out of these crystals there is still dissolved alumina in the liquor at a A/C ratio of approximately 0.3. This liquor is retured to the original concentration autoclave.
The term fine crystals is here meant to cover crystals of which no more than 10% have a mesh size greater than 74 microns, no more than 50% have a mesh size between 44 microns and 74 microns, and the remainder have a mesh size smaller than 44 microns. The term medium-coarse crystals here is intended to cover crystals of which no more than 20% have a mesh size greater than 74 microns, approximately 55% have a mesh size between 44 microns and 74 microns, and the remainder have a mesh size less than 44 microns. Coarse crystals refers to crystals of which approximately 25% have a mesh size greater than 74 microns, approximately 65% have a mesh size lying between 44 microns and 74 microns, and the remainder have a mesh size smaller than 44 microns.
The European method produces a large quantity of fine alumina crystals which can be calcined to give anhydrous alumina that is then reduced to form aluminum. The handling of such fine crystals as well as their transporting is rather difficult. Such a fine aluminum oxide therefore has considerable losses in the form of dust and in subsequent electrolytic treatment the fine crystals do not take on the fluorine well. Since fluorine is an extremely toxic element such losses of fluorine gas must be avoided at all cost.
The American method produces substantially coarser crystals, but it is necessary to use a relatively thin lye solution. Thus, it is necessary to work a large quantity of liquid in accordance with the American process so that a considerable equipment expense and complex procedures must be employed.
It has been suggested to improve on the American method by providing a row of separator-agitator tanks and classifiers so as to produce a relatively coarse aluminum oxide hydrate. In such an arrangement, although the product is superior, again a large equipment cost is encountered.
Cooling of the lye has also been suggested to make the separation of additional aluminum oxide possible. However, a great amount of very fine aluminum oxide trihydrate crystals is produced in accordance with this method, and these crystals are not suitable for the commercial production of aluminum oxide or even for use as seed crystals. Another disadvantage of this method is that additional separator-agitator tanks must be provided to precipitate additional hydrate crystals from the cooled lye and special filters, concentration, and sedimentation tanks are required to remove the fine solids from the lye solution.
Coarse aluminum oxide is also produced according to a method in which extremely fine seeding crystals are prepared in a separate process. Such a method only produces a limited quantity of coarse crystals and requires an entirely separate plant to form the necessarily fine seed crystals that are added to the fresh mother liquor. In addition it is necessary with this method to use a larger number of separator-agitator tanks.
Various systems for extraction alumina from bauxite are described in the U.S. Pat. Nos. 2,707,669, 3,543,923, and 3,632,310.